Boston terrorist bombing: Heat, pressure and shrapnel that can travel faster than a bullet.

This FBI photo shows the remains of a pressure cooker that the agency says was part of one of the bombs that exploded during the Boston Marathon. The FBI says it has evidence that indicates one of the bombs was contained in a pressure cooker with nails and ball bearings, and it was hidden in a backpack.

We’ve all seen the Boston Marathon explosions. But even agonizingly slow-motion replay upon replay of the video snatches cannot begin to reveal what was happening in those combustive seconds.

Let me tell you the physics of it, the god-awful ruin of it against human flesh.

Explosive material packed around a device — maybe Semtex, perhaps some other product — releases an eruption of energy that heats up the bomb’s contents.

In the blink of an eye, those contents and their casing, six-litre pressure cookers according to investigators, but it could be a sealed shut length of pipe, an artillery shell, even a coffee urn or pop bottle, are pushed outward on a pressure wave.

Think of water boiling inside a lidded pot, the steam looking for a means to escape.

This super-hot, highly compressed air will melt and warp everything in its vicinity, air particles travelling faster than the speed of sound. The blast wave lasts only a matter of milliseconds, dissipating quickly and lessening with distance, but often inflicts the most damage at point of impact and those in the immediate vicinity.

The blast wave’s full force is determined by the quantity of the igniting explosive. An average bomb — the type strapped around a suicide bomber’s chest, for instance (although suicide bombers have been ruled out in Monday’s atrocity) — detonates at a rate of about 28,000 feet per second, 22 times faster than a 9mm bullet leaving the muzzle of an automatic hand gun.

Surrounding air pressure, normally 15 pounds per square inch, spikes to 2,200 pounds per square each. That level of heat and pressure will melt iron.

The bombs that ripped through the Boston Marathon crowd appear to have been fashioned out of ordinary kitchen pressure cookers, packed with nails and other fiendishly lethal shrapnel.

Anyone standing nearby would feel, momentarily, a shock wave slamming his or her chest that can rupture organs — liver, spleen, hear and lungs, pulling them away from surrounding tissue — and liquefy eyes. Limbs are ripped off, hands separated from arms and legs from hips.

A physics professor once told me that the effect on an exposed part of the body would be worse than falling off a building.

The initial explosion is followed by shockwaves, blast wind moving at 100 miles an hour, like a mini cyclone. The fragmentation from any additional shrapnel is thrown out violently. The heat will char skin. The shockwave will pick up other objects near by and they become deadly fragments as well.

Even a crude bomb, made according to instructions that can be pulled off the Internet, using fertilizer as explosive material, dumped into a waste bin, activated from a distance or by an attached timer, a wristwatch alarm, can cause extraordinary havoc.

Canadians who were in Boston describe the scene immediatley following the blasts.

Three dead, upwards of 175 injured — wounded would be the more proper term, because this was no accident but an act of aggression – many of them critically, requiring the amputation of shredded limbs, finishing off, as one surgeon noted sadly, what the bomb blast had begun.

“Many of them have severe wounds, mostly in the lower part of their body, wounds related to the blast effect of the bomb as well as small metallic fragments that entered the body, pellets, shrapnel, nails,’’ Dr. George Velmahos, chief of trauma surgery at Massachusetts General Hospital told reporters Tuesday.

“There are a variety of sharp objects that we found in their bodies. Probably this bomb had multiple metallic fragments in them and we removed pellets and nails. I think they came from the bomb although I cannot be exactly sure.

“This bomb obviously was placed probably low on the ground and therefore lower extremity injuries are to be expected.’’

Placed low on the ground, with the cruel irony — perhaps intended but no one knows at this moment — of mangling runners’ legs.

It does not appear, in this early stage of investigation, that this attack was the handiwork of foreign terrorist elements. It lacks the earmarks of an Al Qaeda or even Taliban assault in the clumsiness of the event and the device. Al Qaeda may have been degraded by worldwide anti-terrorism efforts but they’re still capable of far worse than this. Nor does the fact it occurred on Patriot’s Day, at the Boston Marathon, suggest foreign-plotted symbiosis. It feels American.

That’s all speculation, of course. But the abomination of this act seems more likely to have arisen from either domestic terrorism — Lord knows the United States has spawned any number of violent, villainous home-grown groups — or the nihilistic fury of a twisted individual.

In fact, had this been an attack planned and executed by foreign terrorist groups, it would likely be easier for experts to solve. Career bombers and those elite professionals who train others inevitably leave a signature, often in the means of wiring or the materials used. Those are always crucial clues. The purchase of signature materials can be traced, given the vast investigative tools available to intelligence agencies.

But one malevolent person, a rookie using common materials easily accessed, can more easily elude discovery, unless somebody with information spills.

It was, however, clearly an act of terrorism, intended not only to kill and maim but induce broad-based fear. Even sophisticated terrorist acts can’t topple a regime or force its ideology on a populace; it can and does spawn, well, terror. That’s the point.

To his discredit, President Barack Obama didn’t call it what it was in his original comments on Monday. So clearly averse was Obama to the implications that might have been spawned by articulating what everyone was worrying about in their bones — another foreign attack on American soil — that he ducked the T-word completely. That made him look foolish and cowed, rather than commanding and resolute — the president who was such a comfort and rallying figure in those terrible hours after the Newtown shooting tragedy.

He urged Americans not to jump to conclusions, without ever stating what those conclusions so palpably would be. He could have easily done both.

Instead, it took almost 24 hours for Obama to get it right, after other officials and politicians, including Democrats, had publicly stated it. This is precisely why the president was criticized, correctly, for avoiding the terrorism term in the immediate aftermath of last year’s attack on the U.S. compound in Benghazi, on the anniversary of 9-11. He waited a day, even as administration officials were promoting an alternate narrative; that the assault had been spontaneous and triggered by an angry mob.

During his first term in office, the president excised the phrase “war on terror,” although the U.S. is obviously conducting one, most controversially in its use of drone attacks. Now he seems to be preoccupied with striking “terrorism’’ out of the vernacular as well.

Belatedly, Obama declared on Tuesday: “Given what we now know of what took place, the FBI is investigating it as an act of terrorism.’’

It was not his finest hour.

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